A multilayer printed wiring board is known as a technology capable of achieving a high-density mounting of parts and of connecting the parts in the shortest distance. An interstitial via hole (IVH) is a technology which is applied to the production of a multilayer printed wiring board for which higher-density mounting is required, and is characterized by filling a conductive material into a hole (via hole) opened between neighboring layers to electrically connect the circuits of the neighboring layers each other. According to IVH, since interlayer connection can be formed only at necessary portions and a part can be mounted also on the via hole, it becomes possible to achieve a high-density wiring having a high degree of freedom.
As the conductive paste to be filled into such a via hole, there has been used one wherein conductive particles are kneaded into a low-viscosity resin, e.g., a low-viscosity epoxy resin. For example, JP-A-2003-92024 (Patent Document 1) discloses “a via hole-filling conductive paste composition wherein a conductive powder obtained by mixing a plurality of conductive powders having an average particle diameter ranging from 0.5 μm to 20 μm and having different average particle diameters (=conductive particles) is dispersed into a liquid epoxy resin (=a low-viscosity epoxy resin)”. In the conductive paste, in order to solve the problem of generation of concavity at filling, conductive particles obtained by mixing two or more conductive powders having different average particle diameters are used.
As the conductive particles to be mixed with such a conductive paste, a flat filler resulting a high conductivity is preferably used. However, the conductive paste wherein a flat filler is used as conductive particles and it is kneaded into a low-viscosity resin, e.g., a low-viscosity epoxy resin suffers from the problem that filling ability into via holes is poor and bubbles remain in via holes after filling.
When the filling ability into a via hole becomes worse, insufficient connection between the circuits and the like may be induced and reliability of the connection may be lowered. In particular, recently, with the requirement of miniaturization of electronic parts, a multilayer printed wiring board having higher density is required and a via hole having smaller diameter is desired. As a result, the problem that the filling of the via hole becomes more difficult and the reliability of the connection is lowered by the decrease of the filling ability tends to become more remarkable.
Moreover, a multilayer printed wiring board wherein the electrical connection between the circuits is achieved by the filling of the conductive paste into the via hole is sometimes placed under an environment of high temperature and high humidity for a long period of time and it is desired that the electrical resistance (connection resistance) at the connected portion does not change with time. When the connection resistance changes with time, the reliability of the connection is lowered.
Therefore, it is desired to develop a conductive paste which has a good filling ability into a via hole and which can form a connected portion (via hole) with no change in connection resistance with time even under an environment of high temperature and high humidity.    Patent Document 1: JP-A-2003-92024